1. Field of the Invention
The present invention concerns a pulse measurement device. More particularly, it concerns an improved, portable, wearable mechanism for obtaining a relatively noise-free measurement of human pulse rate, under conditions of motion or other activity that tends to create severe noise in the signal. uch a device may be termed a "wristwatch pulsemeter."
2. Brief Discussion of the Prior Art
Several devices have been proposed for providing a wristwatch type of pulsemeter. One type of such device is the digital plethysmograph described in Prinz U.S. Pat. No. 4,120,296, which customarily utilizes an infrared light transducer. Others have proposed using piezoelectric or other pressure sensitive transducers, such as in Stupay U.S. Pat. No. 4,059,118, which uses an "actuator" pin pressing against a piezoelectric crystal.
Typically, such devices tend to have several shortcomings. Those using an optical transducer, such as the digital plethysmographs, consume substantial power in their light-emitting elements, and thus use up their battery life rapidly. Those devices suggesting use of piezoelectric transducers, such as Stupay, typically devote little attention to the substantial noise problem that attends use of such transducers in this application.
When such a pulsemeter is mounted on the wearer's body, as in a wristwatch device, the pulse signal is to a substantial extent masked by concurrent noise signals due to body motions. The mechanical transducer responds both to pressure from the wearer's pulse beat and to motion from walking, arm swinging, and the like, and does not distinguish between them. The latter is noise, however, insofar as pulse measurement is concerned. Thus, the Stupay patent notes that "the patient must remain quiet to avoid noise input" during the period in which the pulse rate is measured.
Also, if the piezoelectric transducer is not mounted directly over the artery of the user, the pulse signal measured by such devices is of considerably lessened amplitude, and is thus even more likely to be masked by noise. Typically, noise signals may be as high as as 1.0 volts, while the pulse signal may be approximately 0.1 volts. Prior art wristwatch pulsemeters of the piezoelectric transducer type have been inaccurate because of this very unfavorable signal-to-noise ratio. Thus, Cramer U.S. Pat. No. 4,224,948 states that when a piezoelectric transducer is used, "the watch must be worn on the volar surface of the wrist but lateral to the tendon cord bundles," so as to obtain a pulse reading from the radial artery in the subpollex depression; further, "the sensors must be forced into the flesh of the wrist for a reading" and "[t]his situation may be uncomfortable."
Insofar as the inventor is aware, the prior art does not disclose an effective noise-free piezoelectric transducer mechanism for human or animal pulse measurement, which obviates the difficulties described above.